A rotating shaft in a turbomachine, such as a turbo-compressor or turbine, is typically mounted using two radial bearings near the ends of the shaft. Two types of radial bearings that are primarily used are magnetic bearings and oil film bearings. For most turbomachine applications, however, magnetic bearings provide the potential to offer superior performance over oil film bearings, as magnetic bearings provide adequate stiffness, damping, and load capacity, with a reduced power consumption due to reduced viscous and windage losses. Moreover, oil film bearings require lubrication and auxiliary systems such as valves, pumps, filters, coolers, and the like, all of which pose a risk of process contamination.
Magnetic bearings can be either active or passive. Active magnetic bearings typically include electromagnetic coils surrounding a ferromagnetic shaft, shaft position sensors, and a control system. In response to signals received from the shaft position sensors, the control system adjusts the electrical current supplied to the electromagnetic coils, thereby adjusting a magnetic field that levitates and/or centralizes the ferromagnetic shaft. Passive magnetic bearings, on the other hand, use permanent magnets rather than electromagnets to position and support the shaft and therefore depend heavily on permanent ferromagnetic attraction or repulsion forces. The stiffness and damping of passive magnetic bearings, however, are much less than active magnetic bearings and they are inherently unstable since it is nearly impossible to stably support a shaft using only passive magnetic bearings. Therefore, in general, active magnetic bearings are usually utilized in addition to the permanent magnet bearings in the axial and radial direction.
Adequate bearing support for the shaft may be utilized to prevent contact between the stationary and rotating components of the turbomachine. Also, appropriate bearing support may be provided to control the deflection of the shaft throughout the operating speed range of the turbomachine. To increase turbomachine performance, the shaft length may be increased to accommodate more rotating assemblage, such as impellers or wheels. Typically, the shaft length is increased by joining two shafts end to end using a tie-bolted joint or the like. However, increasing the shaft length between the radial bearings and adding more impellers or wheels may make the rotordynamics of the rotor more challenging to control and may result in increased shaft vibration/deflection that causes contact between the rotating and stationary components of the turbomachine resulting in damage.
Accordingly, there is a need for a magnetic bearing system that provides additional shaft support (for example, stiffness and damping) between the two shaft bearings to provide the capability of implementing more impellers or wheels without adversely affecting turbomachine rotordynamics, and increases the stiffness and compactness of the joint between the two shafts.